Online casinos have several motives for offering these. For this reason, we should regard them as a kind of 'test drive' of a new casino: we can try out its games and service without committing a deposit. They appear in the form of credit or free spins, without the player having to deposit any money, as the name suggests.
Bonuses are very popular among online casino players, and no deposit bonuses are the closest thing to receiving something for nothing.